QS138 90H V3 Max Voltage

[tonythedrony]

Hello all.

I am currently planning to build an electric go kart, and am planning to use a QS138 90H V3 motor (with the internal gearbox). It's rated voltage is 72V. What would be the performance I would be getting out of such a setup?

Also, would it be possible to increase the voltage to get more performance out of the motor without sacrificing reliability? I saw this thread where the motor was pushed all the way to 110V and yielded good results.

Thanks.

 

[Hummina Shadeeba]

I have it doing 126v. I’ve seen a graph by QS at that speed but can’t find it now.

 

[stancecoke]

Also, would it be possible to increase the voltage to get more performance

What is your aim? Higher voltage only increases the maximum rpm. For more torque you'll need more motor amps. The controller defines the maximum motor current.
mechanical power is proportional to motor current*rpm.

regards
stancecoke

 

[gobi]

What determines the max voltage for a motor? the thickness of the coating of the copper in the windings?

 

[Hummina Shadeeba]

you can get more torque with higher voltage with this and any other mid-drive when you add more gearing converting the speed to torque.

max voltage of a motor is wholly limited by how fast the motor can safely spin before it flies apart or the bearings fail. The enamel on the magnet wire can take thousands of volts.

 

[E-HP]

What is your aim? Higher voltage only increases the maximum rpm. For more torque you'll need more motor amps. The controller defines the maximum motor current.
mechanical power is proportional to motor current*rpm.

regards
stancecoke

It depends on where you want the torque. In the lower RPM range, where the phase current limit is below what the motor can take, torque will be capped at the same level on the lower and higher voltage systems. But at higher RPMs, where the limit isn't affecting/capping the torque, the higher voltage system will have a big advantage. So, if you want to accelerate harder between 20 and 30 for instance, the advantage goes to the higher voltage system.

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[Hummina Shadeeba]

In the lower RPM range, where the phase current limit is below what the motor can take, torque will be capped at the same level on the lower and higher voltage systems

With bigger gear ratio torque is increased at all speeds.

 

[stancecoke]

It depends on where you want the torque

The torque is always defined by the motor current, independent from speed. That's the basic physics of a motor.
At some speed, the BEMF will get too high, so the voltage difference between BEMF and battery gets too low to push the wanted motor current through the coils. Only in this case a higher voltage will be useful. Of course everything is limited by the maximum current, your battery can deliver.

battery current = duty cycle * motor current

 

[E-HP]

With bigger gear ratio torque is increased at all speeds.

Agreed. Gearing or higher voltage provides usable torque at higher speeds. I’d argue that is where it’s needed most, since high torque off the line is basically useless since anything above 90NM or so just makes you flip over on your back.

 

[SlowCo]

Why use a motor version with build in reduction on a go kart with small wheels? I suspect it will be better to use the QS180 motor without reduction and use direct drive with a chain/sprocket final reduction.

 

[Hummina Shadeeba]

Gearing or higher voltage provides usable torque at higher speeds

bigger gearing provides higher torque at all speeds. If u had 75v and swap to 100v and also increase the gear ratio by 25% you’ve now swapped that 25% extra speed for 25% more torque regardless of what speed the motor spins

On a kart with such small wheels and room to add a big sprocket the internal gear seems unnecessary as said.

 

[E-HP]

bigger gearing provides higher torque at all speeds. If u had 75v and swap to 100v and also increase the gear ratio by 25% you’ve now swapped that 25% extra speed for 25% more torque regardless of what speed the motor spins

On a kart with such small wheels and room to add a big sprocket the internal gear seems unnecessary as said.

Im not saying upping the voltage is the best way to get there, but the OP’s question was:
“Also, would it be possible to increase the voltage to get more performance out of the motor without sacrificing reliability?”
Which I believe is yes. Your numbers make a convincing argument for the best method to increase performance, so makes a lot of sense, if that were the question.

 

[amberwolf]

What determines the max voltage for a motor? the thickness of the coating of the copper in the windings?

Yes, or more generally the insulation thickness/capability of all the wiring in there.

I expect that most of these motors, assuming completley undamaged enamel, would handle at least a couple of hundred volts easy, and possibly a lot more.***


Another consideration is motor kV--if you have a motor meant to spin at say, 300RPM at 36v, that's a kV of about 8.3, I think. So say, 200v at 8.3rpm/v gives a full-throttle speed of about 1600rpm.... (i'm terrible with math, so this could be wrong)


***if the enamel is damaged, voltage tolerance can drop a lot. I have an MXUS 450x hubmotor that had damaged winding enamel at a stator tooth edge (I think), that showed no problems until voltage rose enough while driving it with an FOC controller on a 14s 52v battery, and then all sorts of weirdness would ensue. On a typical generic controller, it was fine. (why? dunno--probably because the FOC would be monitoring phase currents cycle by cycle and error out/etc when this happened, as it would cause wrong current flows...but the generic didnt monitor phase currents, so it would just shove it along anyway). I found the problem by using a "hipot" tester, and I fixed the motor by drizzling CoronaDope down into the windings and shoving some ziptie remnants in between them and the laminations near the problem area.

 

[bunya]

The other consideration is iron losses which increase with voltage/rpm. I'm build a KTM EXC motorbike with this motor at 28s. It think the point of max performance would be somewhere around 32s. But you'd probably overheat the motor running at high speeds around the race track for too long.

 

[Hummina Shadeeba]

what’s the mechanical limit beyond which the speed of the rotor, or gears, or bearings, causes them to break?

What of the possibility of running it with 200 volts? It won’t be efficient at high speeds but if trying to get maximum power from as small a motor as possible for a short period, like a drag race, why not?

 

[ray88]

Easy 140-160V DC without field weakening.. It has been done by one guy from Germany but with V1